Metformin is the most widely prescribed drug for the treatment of Type II diabetes. It was approved for marketing by the U.S. Food and Drug Administration in 1994, yet the mechanisms involved in its intestinal absorption and its pharmacologic actions are not well understood to date. Despite its hydrophilic nature with net positive charge at physiological pH values, metformin exhibits high oral bioavailability (50-60%) with wide inter-subject variability. It is ineffective in 38% of patients, and can cause severe lactic acidosis in some. A major goal of the proposed research is to identify and characterize the intestinal transporter(s) and absorption mechanism of metformin, and elucidate their role in its pharmacologic actions. Recent observations in the PI's laboratory show that metformin is taken up across the apical cell membrane into Caco-2 cell monolayers, a well-established model for human intestinal epithelium, via the cation-selective tranaporters OCT1 and PMAT. Once inside the cell, metformin cannot egress across the basolateral membrane, resulting in intracellular accumulation during its absorptive transport. To understand the implications of these observations in the clinic, an experimental approach has been designed to achieve the following objectives: (i) Test if the mechanism of metformin transport observed in an in vitro static Caco-2 cell model applies to its transport behavior in the intestinal epithelium from mouse and humans. (ii) Elucidate the relationship between metformin transport mechanism in the mouse intestinal epithelium to its intestinal absorption, its pharmacologic profile, and adverse effects in a diabetic mouse model. (iii) Apply these results to retrospectively evaluate the highly variable oral bioavailability in humans, the lack of pharmacologic response to metformin in some, and the incidences of lactic acidosis in a small number of patients. . The proposed research addresses the gap in our knowledge regarding the pharmacokinetic and pharmacologic behavior of metformin. The results from the proposed work will improve understanding of the absorption mechanisms of other hydrophilic cationic drugs. The results will also provide a mechanistic basis for (i) inter- individual variability in therapeutic response to metformin and (ii) gastrointestinal side effects as well as lactic acidosis. Metformin analogs or prodrugs with improved therapeutic profile can be conceived based on the findings of this research